Written explanation to Gary ricketson's animated gif along with my own info:

1.Synaptic is opened from menu -----> system -------> synaptic or from the command line after becoming root by typing synaptic2. File ------> History ; will open a new dialog box which shows the list of packages that were installed.3. Help ---> icon legend will show a list of icons and what that means for a particular package.4. To search a package click the search icon and type the package name and hit enter.5. To install select the package and hit enter and click apply or right click and from the menu select mark for installation and then click apply6. To lock a package with particular version goto package-----> lock version, for that package an icon with lock will appear after that. you will not be able to upgrade that package. It is similar to apt-pinning.7. To fix broken packages goto Edit ------> Fix broken packages8. To save the packages for later installation goto file-----> save markings as and save it.9.To install the saved markings goto File-----> Read markings as and select the file with markings then click apply.10. You can also generate a download script to install packages goto File---> Generate package download script and save the script. Run the script to download the packages. 11. Goto File-----> Add downloaded packages to install the downloaded packages using the script. This is particularly useful for computers with slow or no internet connection12. To install only the dependencies goto preferences ---> in General tab Tick off the option consider recommended packages as dependencies13. The Reload option will refresh the repo for updated packages;after refreshing the repo click mark all upgrades to upgrade the packages.14. To download the changelog of a particular package goto Package----->Download changelog it will give you the changelogs in a diaglog box.